The effect of selenium compounds on biological systems has been of great interest to researchers. Although selenium is toxic if taken in high doses, it is an essential trace element in a healthy diet. Various selenium-based compounds, such as selenium sulfides, have long been recognized for therapeutic applications in the topical treatments for dandruff, eczemas and dermatomycoses. Researchers have also explored the use of selenium based compounds in nutritional supplements for the protection of normal cells in the prevention of cancer, the inhibition of aging processes, and the replication of HIV in AIDS patients. More recently, research has focused on the use of selenium compounds as cytotoxic agents in the treatment of cancer.
Like most group VI elements, selenium occurs in different oxidation states (−II, 0, +II, +IV, and +VI). Up to now, all biologically active selenium compounds were derived from selenium in oxidation states −II, +II, +IV, or +VI. Selenium having an oxidation state zero (Se(0)) however, was believed to be biologically inert.